The present invention relates generally to the manufacturing of steel objects from steel powders, and more specifically to bearings fabricated from highly alloyed steel powders supersaturated in carbon up to 0.5 weight percent in ferrite, and up to 1.0 weight percent in austentite type combinations, using a rapid solidification process.
Heretofore, bearings used for high performance gas turbine engines were manufactured from alloy steel ingots made by either the consumable electrode vacuum melt process (CEVM) or the vacuum induction melting vacuum arc refining process (VIM-VAR). Both these processes make extremely clean steel ingots of high quality without silicate inclusions, suitable for bearings and gears, but these ingots are slowly cooled and result in carbon segregration leading to large complex carbide particles in the starting materials from which the bearings are subsequently fabricated. These coarse complex carbides are detrimental to the operating lifetime of a bearing since they lead to corrosion problems associated with pitting, and a reduction in fatigue lifetime associated with premature crack nucleation at large carbide particles.
The present invention proposes to produce bearings with very high uniformity of structure, on a scale less than one micron, using a rapid solidification process to make rapidly solidified powder, and then to consolidate this powder by thermomechanical processing for bearing components. The task of fabricating bearings from rapidly solidified steel powders is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 3,556,780 issued to F.C. Holtz, Jr; PA1 U.S. Pat. No. 3,834,004 issued to M. D. Ayers; PA1 U.S. Pat. No. 4,050,143 issued to C. Aslund; PA1 U.S. Pat. No. 3,936,299 issued to G. Steven; PA1 U.S. Pat. No. 4,078,873 issued to P. Holiday et al; and PA1 U.S. Pat. No. 4,355,057 issued to E. R. Slaughter.
All of the above references are exemplary in the art and disclose applications of powder metallurgy which result in the manufacture of steel objects by compressing steel powder. The Holtz and Ayers references describe the fundamentals of powder metallurgy, including the process of atomization of melted alloys to produce alloy powders; and the consolidation of alloy powders into solid stock using pressures from 5-30 tons per square inch, and forging temperatures of up to 2,150 degrees F.
The Aslund and Steven references further disclose the production of tool steel articles, including dense metal tubes, from stock produced from metal powders. The disclosure of Holiday et al is useful for its description of the atomizing apparatus, which is used to produce metal powders. Finally, Slaughter discloses the formation of alloy powders using rapid solidification techniques.
While the above-referenced disclosures are helpful, the bearings currently used in high performance gas turbine engines have predominantly been manufactured from either the consumable electrode vacuum melt process, or the vacuum induction melting-vacuum arc refining process, which produces stock with coarse complex carbides which lead to corrosion problems and attenuate the operating lifetime of bearing elements. From the foregoing discussion, it is apparent that there currently exists the need to fabricate bearings from alloyed steel powders to improve the carbide phase characteristics. The present invention is intended to satisfy that need.